This sentence uses the incorrect form of "who." "Who's" is a contraction for "who is." The appropriate form here is whose, which is the possessive form of the word "who." The correct choice is, "to know whose letter she received."

This sentence misuses the word "who." The pronoun "who" is reserved for humans only. All other things, whether living or not, is referred to as "which" in standard English. Note that "that" is inappropriate here because it would make the following phrase ("the company built") a part of the main clause, which would mean you would have to omit the commas. The same explanation applies to just omitting the pronoun. The correct choice is, "The robot, which the company built, was the first."

This sentence misuses the pronoun "who's." There are two issues with the usage of this word: first, "who's" is a contraction for "who is" and is not possessive; the correct form would be "whose." The second issue, however, is that "who" should never be used with non-human words. The correct pronoun would be "which." "That" cannot be used because it would make the phrase "has products I love" a part of the main clause, which would require deleting the commas. The correct choice is, "which has products I love."

This sentence misuses the pronoun "who's," which is a contraction for "who is." It is clear from the context of this sentence that the possessive form, "whose," is necessary here. The other choice are not actual forms of this pronoun or are too wordy. The correct answer is, "whose."

This sentence misuses the word "it's," which is a contraction for "it is." The appropriate form here (the possessive) is "its," omitting the apostrophe. No additional changes are necessary. The correct choice is, "to have a problem with its transmission."

This sentence misuses the pronoun "who." This pronoun is only used when referring to humans. The correct pronoun for a robot is "which." Note that "that" could also be appropriate if the comma before the word were omitted. Clauses that begin with "that" are not separated with commas, but relative clauses (Those beginning with forms of "who" or "which") should be separated by commas. The correct choice is, "the robot, which had the capability to run."

This sentence misuses the word "who's," which is a contraction for "who is." The context of the sentence tells us that the pronoun should be possessive. The correct form is "whose."

This sentence misuses the word "that." The word "that" does not begin a new clause; therefore, there should not be a comma before the word. Either "that" needs to be changed to "which," or the comma needs to be omitted. The choice that corrects this issue is, "areas, which ultimately led to her new invention."

This sentence uses the wrong pronoun. The word "which" is never used for nouns which refer to humans. The correct choice here is "who." No other changes are necessary. Note that "that" does not begin a relative clause (in which case you would have to omit the commas), and the word "being" generally has to do with existence and is never used in this way. The correct choice is, "the teens, who were excited, bought much food."

This sentence misuses the word "which." Since the sentence is about the Presidents (this is the subject), "which" is inappropriate as it can only refer to non-humans (Additionally, nations cannot "convene," but representatives (i.e. people) from those nations can). If the relative clause were to be re-written, it would be "they convened to talk about world issues." Since "they" is acting as the subject here, the correct word to replace it would be "who." The correct choice is, "who."